Lanayru's Lightning Round is two parts - Battle and Silent Realm. This is the former - the Boss Rush, where Link may fight all the bosses he's encountered thus far. If you happen to be playing in Hero Mode, all of them will be unlocked from the start.

Win 4 and 8 battles, get a Piece of Heart and an "absurdly sturdy" shield, respectively, the first time you get to each. Win all 12 on Hero Mode, get a complete refill of your wallet.